The above-mentioned compound [11] useful as a therapeutic agent for diabetes, an intermediate and a method for producing them have been already disclosed in the specification of WO95/18125, and an intermediate compound [6′]and a method for producing same have been specifically disclosed in Journal of Medicinal Chemistry, 1992, Vol. 35, No. 14,2625.
However, these conventional production methods require many steps and the yields of the final product and intermediates therefor are not sufficiently satisfactory. In addition, solvent, base, catalyst and the like to be used in each step suffice for use at laboratory levels but many of them are problematically impractical and cannot be used in industrial production.